In the battery art, particularly research batteries of the sodium/sulfur or lithium/metal sulfide types, the materials used must be of high purity. The battery typically is "sealed" against the atmosphere to prevent atmospheric contamination and loss of active materials. Normally, the battery construction provides for many cells, each having positive and negative electrode assemblies and a common electrolyte, sealed in a housing, and external cell terminals from the housing. A battery case encloses all of the cell housings, and main positive and negative battery terminals are externally exposed relative to the case to allow for appropriate connections to be made with the apparatus powered by the battery. Each cell generates only a volt or two of electrical potential across the cell terminals, so that electrical connections are made externally of the cell housings, but internally of the battery case, in series and/or parallel, between the positive and negative cell terminals and the main positive and negative battery terminals for accumulating a battery voltage of possibly 100 volts or higher.
In most cell constructions, at least the positive or negative terminal of each cell must be passed through an opening in the cell housing, and an insulating and sealing structure (or "feedthrough" seal) is required at the terminal to maintain the cell interior sealed from or isolated from outside the housing, and to electrically isolate the cell housing from the terminal which are at different electrical potentials.
Specifically, the feedthrough seal electrically isolates the cell terminal extended through the cell housing wall from the wall and further is needed because (1) the liquid electrolyte is free to migrate within the cell and/or is exposed to the cell terminal at the underside of the feedthrough seal; (2) the active materials of the positive and negative electrodes must be sealed within the cell housing to minimize contamination to or from the surroundings or the feedthrough seal itself; and (3) the elevated operating temperatures (of the order of 400.degree.-500.degree. C.) can create gases to pressurize the sealed cell chamber. The feedthrough seal thus must be sound and resistant against the temperatures and corrosive nature of the battery materials, and further must preclude the movement of gases past the seal to ensure long term operation of the battery.
A typical lithium/iron sulfide cell or battery has a durable sealed housing; a positive electrode assembly of an iron sulfide (FeS) or iron disulfide (FeS.sub.2); a negative electrode assembly of a lithium aluminum (LiAl) or lithium silicon (LiSi) alloy, with possible secondary additives of iron (Fe) or magnesium oxide (MgO); and a molten electrolyte of lithium chloride and potassium chloride (LiCl-KCl). A porous fiber-like separator of boron nitride (BN) or a powder separator of magnesium oxide (MgO) is typically interposed between the positive and negative electrode assemblies for electrically isolating one from the other. The separator also acts as a reservoir for the electrolyte which provides for the lithium ion transport.
A common feedthrough seal for the positive and/or negative cell terminal of each cell has a lower insulator sleeve, generally formed of beryllium oxide (BeO) fitted over the cell terminal, generally formed of iron (Fe) or nickel (Ni) for the FeS cell, and molybdenum (Mo) for the FeS.sub.2 cell, and into an opening in the housing wall, so that the terminal is electrically isolated from the housing wall. Boron nitride (BN) powder is then tightly packed into an annular cavity formed between the cell terminal and an upstanding housing wall surrounding the terminal. An insulator ring, generally formed of beryllium oxide (BeO) or alumina (Al.sub.2 O.sub.3), is then positioned over the conductive terminal and against the upper side of the boron nitride powder; and the upstanding housing wall is typically then mechanically crimped tightly over the ring. As the boron nitride powder is not wetted by the electrolyte, the seal does prevent electrolyte leakage and does perform quite well considering its need for providing electrical insulation and resistance against corrosion.
One major drawback with this seal construction is the lack of any hard connection between the terminal and the housing, which can raise mechanical durability problems. Still further, boron nitride has to be of a very high purity (equivalent almost to a laboratory grade obtained such as by firing at 1700.degree.-2100.degree. C. in a nitrogen atmosphere for 10 minutes-2 hours), otherwise the cell tends to short out across the boron nitride. Lastly, although both the beryllium oxide and the boron nitride are highly resistant to corrosion, there typically is not sufficient compression of the powdered boron nitride to provide for a completely impervious seal, such as having a leak rate of helium of less than 10.sup.-7 to 10.sup.-9 cm.sup.3 /sec/cm.sup.2. One limiting factor to high compression of the powdered boron nitride is the fact that the beryllium oxide insulator is very brittle and will crack upon excess pressure being directed thereagainst.